Method of forming Si-O containing coatings

Abstract: Disclosed is a method for forming improved Si-O containing coatings on electronic substrates. The method comprises treating Si-O containing ceramic coatings derived from hydrogen silsesquioxane resin with hydrogen gas. The resultant coatings have improved properties such as stable dielectric constants.

Description: The present invention relates to a method of treating Si-O containing ceramic coatings derived from hydrogen silsesquioxane resin (H-resin) with hydrogen gas. The resultant coatings have desirable properties. The use of silica-containing ceramic coatings derived from H-resin on electronic devices is known in the art. For instance, U.S. Patent No. 4,756,977 describes processes for forming such coatings which comprise diluting hydrogen silsesquioxane in solvents, applying the solutions to substrates, evaporating the solvents and heating the coated substrates to a temperature of 150 to 1000°C. in air. This patent, however, does not describe the effects of annealing the coatings with hydrogen gas or the impact on electrical properties. The conversion of H-resin to an Si-O containing ceramic in other environments is also known in the art. For instance, European Patent Application No. 90311008.8 teaches the conversion of H-resin in an inert gas atmosphere. This document also does not describe annealing the coatings with hydrogen gas or the impact on electrical properties. The present inventor has now found that when Si-O containing coatings derived from H-resin are treated with hydrogen gas, the properties of the coating are improved. The present invention provides a method of forming an Si-O containing coating on an electronic substrate. The method comprises first applying a coating comprising H-resin on an electronic substrate. The coated electronic substrate is then heated to a temperature sufficient to convert the H-resin to an Si-O containing ceramic coating. The Si-O containing ceramic coating is then exposed to an atmosphere containing hydrogen gas for a time and at a temperature sufficient to anneal the coating. The present invention also relates to Si-O containing ceramic coatings having a dielectric constant less than or equal to 4.5. The present invention is based on the unexpected finding that annealing Si-O containing ceramic coatings derived from H-resin with hydrogen gas improves the properties of the coatings. For instance, such an anneal can lower the dielectric constant of the coating and render it stable. Moreover, such treatment may also advantageously affect the physical properties of the coating (e.g., moisture absorption, cracking, etc.) These effects are surprising since it was not known in the art how hydrogen gas would affect the coatings and their resultant properties. Because of these benefits and advantages, the coatings resulting from this invention are particularly valuable on electronic substrates. Such coatings could serve as protective coatings, interlevel dielectric layers, doped dielectric layers to produce transistor like devices, pigment-loaded binder systems containing silicon to produce capacitor and capacitor-like devices, multilayer devices, 3-D devices, silicon-on-insulator devices, coatings for superconductors, super lattice devices and the like. As used herein, the expression "ceramic" is used to describe the hard, Si-O containing coatings obtained after heating H-resin. These coatings may contain both silica (SiO 2 ) materials as well as silica-like materials (eg., SiO, Si 2 O 3 , etc.) which are not fully free of residual carbon, silanol (Si-OH) and/or hydrogen. The coatings may also be doped with boron or phosphorous. The expression "electronic substrate" includes electronic devices or electronic circuits such as silicon-based devices, gallium-arsenide based devices, focal-plane arrays, opto-electronic devices, photovoltaic cells and optical devices. According to the process of this invention, a coating comprising H-resin is first applied on an electronic substrate. The H-resin which may be used in this process include hydridosiloxane resins of the formula HSi(OH) x (OR) y O z/2 , in which each R is independently an organic group or a substituted organic group which, when bonded to silicon through the oxygen atom, forms a hydrolyzable substituent, x = 0-2, y = 0-2, z = 1-3, x + y + z = 3. Examples of R include alkyls such as methyl, ethyl, propyl and butyl; aryls such as phenyl and alkenyls such as allyl or vinyl. These resins may be fully condensed (HSiO 3/2 ) n or they may be only partially hydrolyzed (i.e., containing some Si-OR) and/or partially condensed (i.e., containing some Si-OH). Although not represented by this structure, these resins may also contain a small number (e.g., less than about 10%) of silicon atoms which have either 0 or 2 hydrogen atoms attached thereto or a small number of SiC bonds due to various factors involved in their formation or handling. Additionally, the resin may also be doped with boron or phosphorous if desired. The above H-resins and methods for their production are known in the art. For example, U.S. Patent No. 3,615,272 teaches the production of a near, fully condensed H-resin (which may contain up to 100-300 ppm silanol) by a process comprising hydrolyzing trichlorosilane in a benzenesulfonic acid hydrate hydrolysis medium and then washing the resultant resin with water or aqueous sulfuric acid. Similarly, U.S. Patent No. 5,010,159 describes an alternative method comprising hydrolyzing hydridosilanes in an arylsulfonic acid hydrate hydrolysis medium to form a resin which is then contacted with a neutralizing agent. Other hydridosiloxane resins, such as those described in U.S. Patent No. 4,999,397, those produced by hydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholic hydrolysis medium or those described in JP-A (Kokai) Nos. 59-178749, 60-86017 and 63-107122 or any other equivalent hydridosiloxane, will also function herein. In a preferred embodiment of the invention, specific molecular weight fractions of the above H-resins may also be used in the claimed process. Such fractions and methods for their preparation are taught in U.S. Patent No. 5,063,267. A preferred fraction comprises material wherein at least 75% of the polymeric species have a molecular weight above 1200 and a more preferred fraction comprises material wherein at least 75% of the polymeric species have a molecular weight between 1200 and 100,000. The H-resin coating material may also contain other ceramic oxide precursors. Examples of such precursors include compounds of various metals like aluminum, titanium, zirconium, tantalum, niobium and/or vanadium, as well as various non-metallic compounds like those of boron or phosphorous, which may be dissolved in solution, hydrolyzed, and subsequently pyrolyzed, at relatively low temperatures and relatively rapid reaction rates to form ceramic oxide coatings. The above ceramic oxide precursor compounds generally have one or more hydrolyzable groups bonded to the above metal or non-metal, depending on the valence of the metal. The number of hydrolyzable groups to be included in these compounds is not critical as long as the compound is soluble in the solvent. Likewise, selection of the exact hydrolyzable substituent is not critical since the substituents are either hydrolyzed or pyrolyzed out of the system. Typical hydrolyzable groups include alkoxy, such as methoxy, propoxy, butoxy and hexoxy; acyloxy, such as acetoxy; or other organic groups bonded to said metal or non-metal through an oxygen such as acetylacetonate. Specific compounds are zirconium tetracetylacetonate, titanium dibutoxy diacetylacetonate, aluminum triacetylacetonate, tetraisobutoxy titanium, B 3 (OCH 3 ) 3 O 3 and P 3 (OCH 2 CH 3 ) 3 O. When H-resin is to be combined with one of the above ceramic oxide precursors, it is used in an amount such that the final ceramic coating contains 70 to 99.9 percent by weight SiO 2 . The H-resin coating material may also contain a platinum, rhodium or copper catalyst to increase the rate and extent of conversion to silica. Generally, any platinum, rhodium or copper compound or complex which can be solubilized will be functional. For instance, a composition such as platinum acetylacetonate, rhodium catalyst RhCl 3 [S(CH 2 CH 2 CH 2 CH 3 ) 2 ] 3 , obtained from Dow Corning Corporation, Midland, Michigan, or cupric naphthenate are useful. These catalysts are added in an amount of between 5 to 1000 ppm of platinum, rhodium or copper based on the weight of H-resin. The H-resin is coated on the desired substrate by any practical means, but a preferred approach uses a solution comprising the H-resin in a suitable solvent. If this solution approach is used, the solution is formed by simply dissolving or suspending the H-resin in a solvent or mixture of solvents. Various facilitating measures such as stirring and/or heat may be used to assist in the dissolution. The solvents which may be used in this method include alcohols such as ethyl or isopropyl, aromatic hydrocarbons such as benzene or toluene, alkanes such as n-heptane or dodecane, ketones, cyclic dimethylpolysiloxanes, esters or glycol ethers, in an amount sufficient to dissolve the above materials to low solids. For instance, enough of the above solvent is included to form a 0.1-50 weight percent solution. The above H-resin solution is then applied to the substrate. Means such as spin, spray, dip or flow coating will all function herein. Following application, the solvent is allowed to evaporate by means such as simple air drying, exposure to an ambient environment or by the application of a vacuum or mild heat. Although the above described methods primarily focus on using a solution approach, one skilled in the art would recognize that other equivalent means of coating (eg., melt coating) would also function in this invention. The coated electronic substrate is then heated to a temperature sufficient to convert the H-resin to an Si-O containing ceramic coating. The temperature used for heating is generally in the range of 50 to 1000°C. The exact temperature, however, will depend on factors such as the pyrolysis atmosphere, heating time and the desired coating. Preferred temperatures are often in the range of 200 to 600°C. Heating is generally conducted for a time sufficient to form the desired Si-O containing ceramic coating. Generally, the heating time is in the range of up to 6 hours. Heating times of less than 2 hours (eg., 0.1-2 hrs.) are generally preferred. The above heating may be conducted at any effective atmospheric pressure from vacuum to superatmospheric and under any effective oxidizing or non-oxidizing gaseous environment such as those comprising air, O 2 , oxygen plasma, ozone, an inert gas (N 2 , etc.), ammonia, amines, moisture and N 2 O. Any method of heating such as the use of a convection oven, rapid thermal processing, hot plate or radiant or microwave energy is generally functional herein. The rate of heating, moreover, is also not critical, but it is preferred to heat as rapidly as possible. The Si-O containing ceramic coating is then exposed to an atmosphere containing hydrogen gas for a time and at a temperature sufficient to anneal the coating (i.e., a forming gas anneal). This exposure can occur immediately after formation of the ceramic coating or, alternatively, the exposure can be at any later time. Generally, this exposure is accomplished by introducing the desired hydrogen gas into a chamber or furnace used for the anneal. The hydrogen gas used herein can be in any concentration practical. For example, concentrations in the range of between 0.01 and 100 volume percent can be used. Obviously, however, the upper limit of the concentration will be determined by the method of use and the explosive nature of hydrogen. Generally, preferred concentrations are in the range of 1 to 30 volume percent. If the hydrogen gas is to be in contact with air, concentrations of 5 volume percent or lower are used. The diluent gas for the hydrogen is likewise not critical. Inert gases such a nitrogen, argon, helium or reactive gases such as air may all be used. As noted above, however, if a reactive gas is used, the concentration of hydrogen must be carefully monitored to prevent explosions. The temperature used during hydrogen gas exposure can also vary over a wide range. Temperatures of from room temperature up to 800°C. are all functional herein. Preferred temperatures are generally in the range of between 200 and 600°C. The time for exposure to the hydrogen gas can also vary over a wide range. Exposure times in the range of minutes to several hours (e.g., 1 minute up to 4 hours) are functional herein. Preferred exposure times are in the range of between 10 minutes and 2 hours. Although not wishing to be bound by theory, the present inventor postulates that the hydrogen gas exposure removes moisture from the coating and treats the coating so as to prevent reentry of said moisture. In this manner, the dielectric constant of the coating is lowered (eg., to the range of 2.8-4.5) and stabilized. Of particular interest is the fact that the resultant coatings can have dielectric constants less than 3.5, often less than 3.2, occasionally less than 3.0 and sometimes less than 2.8 at 1 MHz (eg., 2.7-3.5). The following examples are included so that one skilled in the art may more readily understand the invention. Example 1 Hydrogen silsesquioxane resin (made by the method of U.S. Patent No. 3,615,272) was diluted to 18 wt % in methylisobutylketone. 4 inch (10.2 cm) diameter 1 mOhm-cm, n-type, silicon wafers were coated with this solution by spinning at 3000 RPM for 10 seconds. The coated wafers were then converted to the Si-O containing ceramics in the manner specified in Tables 1 - 3. The resultant coatings were 0.3 micrometers thick. After conversion to ceramic, sample 2 was exposed to hydrogen gas (5 volume percent in nitrogen) at 400°C. for 20 minutes. Table 1 shows the effect of hydrogen gas on resistivity; Table 2 shows the effect of hydrogen gas on dielectric constant; and Table 3 shows the effect of hydrogen gas on mobile charge. As is evident, exposing the coatings to hydrogen gas has a dramatic effect on their electrical properties. Moreover, as shown in Table 4, the dielectric constant of the coatings treated with hydrogen gas remain stable upon exposure to the laboratory ambient over time, whereas the dielectric constant of the untreated coatings increase upon exposure to the laboratory ambient over time. Resistivity and Defect Density (Mean values, 2 iterations, 2 sigma) No Conversion Conditions Resistivity (Ohm-cm) Defects/cm 2 1 400°C. in N 2 for 1 hour 1.31 x 10 13 1.86 2* 400°C. in N 2 for 1 hour 8.04 x 10 11 3.98 3 400°C. in O 2 for 1 hour 6.16 x 10 13 0.71 1 MHz Dielectric Constant and Loss (Mean values, 2 iterations, 2 sigma) No Conversion Conditions Dielectric Constant Loss 1 400°C. in N 2 for 1 hour 3.907 0.0156 2* 400°C. in N 2 for 1 hour 2.968 0.0024 3 400°C. in O 2 for 1 hour 3.552 0.0161 Mobile Charge No Conversion Conditions Mobile Charge 1 400°C. in N 2 for 1 hour 1.2 x 10 12 2 400°C. in N 2 for 1 hour 1.7 x 10 12 3 400°C. in O 2 for 1 hour 2.0 x 10 12 Dielectric Constant (DK) Over Time No DK at Formation DK after 1 Day DK after 160 Days 1 3.907 3.874 4.081 2 2.968 2.973 2.773 Example 2 Hydrogen silsesquioxane resin (made by the method of U.S. Patent No. 3,615,272) was diluted to 22 wt % in methylisobutylketone. 4 inch (10.2 cm) diameter 1 mOhm-cm, n-type, silicon wafers were coated with this solution by spinning at 1500 RPM for 10 seconds. One of the coated wafers was then converted to the Si-O containing ceramics by heating at 400°C. in nitrogen for 1 hour. The resultant coating was about 0.8 micrometers thick. The dielectric constant was 5.8. The coated wafer was then exposed to hydrogen gas (5 volume percent in nitrogen) at 400°C. for 20 minutes. The dielectric constant was decreased to 3.8. A second coated wafer was then converted to the Si-O containing ceramics by heating at 400°C. in oxygen for 1 hour. The resultant coating was about 0.8 micrometers thick. The dielectric constant was 5.0. The coated wafer was then exposed to hydrogen gas (5 volume percent in nitrogen) at 400°C. for 20 minutes. The dielectric constant was decreased to 4.2. As is evident, exposure to hydrogen gas has a significant effect on the dielectric constant of 5.8 to 3.8 and 5.0 to 4.2, respectively.

Claims: A method of forming an Si-O containing coating on an electronic substrate comprising: applying a coating comprising hydrogen silsesquioxane resin on an electronic substrate; heating the coated electronic substrate to convert the hydrogen silsesquioxane resin to an Si-O containing ceramic coating; and exposing the Si-O containing ceramic coating to an annealing atmosphere containing hydrogen gas to anneal the coating. The method of claim 1 wherein the coated substrate is converted to the Si-O containing ceramic coating by heating at a temperature in the range of between 50°C. and 1000°C. for up to 6 hours. The method of claim 1 wherein the hydrogen silsesquioxane resin is fractionated into polymeric species such that at least 75% of the polymeric species have a molecular weight between 1200 and 100,000. The method of claim 1 wherein the hydrogen silsesquioxane resin containing coating also contains modifying ceramic oxide precursors comprising a compound containing an element selected from titanium, zirconium, aluminum, tantalum, vanadium, niobium, boron and phosphorous wherein the compound contains at least one hydrolyzable substituent selected from alkoxy or acyloxy and the compound is present in an amount such that the silica coating contains 0.1 to 30 percent by weight modifying ceramic oxide. The method of claim 1 wherein the hydrogen silsesquioxane resin containing coating also contains a platinum, rhodium or copper catalyst in an amount of between 5 and 1000 ppm platinum, rhodium or copper based on the weight of hydrogen silsesquioxane resin. The method of claim 1 wherein the coated substrate is converted to the Si-O containing ceramic coating by heating in an atmosphere selected from air, O 2 , oxygen plasma, ozone, an inert gas, ammonia, amines, moisture and N 2 O. The method of claim 1 wherein the annealing atmosphere contains hydrogen gas in a concentration in the range of 1 to 30 volume percent. A method of lowering the dielectric constant of an Si-O containing ceramic coating derived from hydrogen silsesquioxane resin which comprises: exposing an Si-O containing ceramic coating derived from hydrogen silsesquioxane resin to an atmosphere containing hydrogen gas and annealing to lower the dielectric constant of the coating. A method according to claim 8 wherein the dielectric constant is lowered to less than 3.2 at 1 MHz. A method according to claim 9 wherein the coating is formed onto an electronic substrate and further wherein the coated substrate is made to form part of an electronic device.

Inventors: BALLANCE DAVID STEPHEN, CAMILLETTI ROBERT CHARLES, DUNN DIANA KAY, BALLANCE, DAVID STEPHEN, CAMILLETTI, ROBERT CHARLES, DUNN, DIANA KAY
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1994
Main classification: H01L21
IPC codes: C04B35, C01B33, H01L21